The present invention relates to an optical fiber sensor device and a vibration position specifying method.
When vibration is applied to an optical fiber from an outside, a polarization state of a light wave output from the optical fiber is temporally changed. A cause for that is a temporal change of birefringence of the optical fiber involved in the vibration (a variation both of the phase difference generated between the birefringent axis and of the birefringent axis), and as a result, a phenomenon of temporal change of the polarization state of the output light wave is observed. Using this phenomenon, by bonding the optical fiber to a fence or the like, inputting probe light to this optical fiber, and observing the temporal change of the polarization state of the output probe light, vibration of the fence or the like can be sensed. So far, as the optical fiber sensor device using this phenomenon of the temporal change of the polarization state of the output light wave, an intrusion detection optical fiber sensor and a rockfall sensing optical fiber sensor are disclosed in JP 2000-40187A (Patent Literature 1).
Moreover, a method of specifying a position of vibration applied to the optical fiber is disclosed in JP 2008-203239A (JP 5242098B; Patent Literature 2). This method is a fluctuation position detecting method in which the optical fiber is bonded in a loop state to an object to be measured such as a fence, polarized probe light is distributed to two parts, the distributed probe lights are led to both ends of the aforementioned loop-state optical fiber and made to propagate in different directions in this optical fiber, a temporal change of the polarization state of each of the output light waves is observed, and a vibration position is specified from a difference in start time of the change of their polarization states.
The light waves input into the optical fiber take time of approximately 5 ns to propagate for 1 m and thus, a difference is generated in the start time of the temporal change of the polarization states of clockwise and counterclockwise propagation lights by the position of vibration applied to the loop-state optical fiber, and the vibration position can be specified on the basis of this difference.